1. Field of the Invention
The present invention relates to an automatic equalizer for automatically equalizing digital signals containing nonlinear distortions and, more particularly, to an automatic equalizer for automatically equalizing a digital signal experiencing from nonlinear distortions, e.g., a digital signal from a read head of a high density magnetic recording apparatus, to thereby faithfully reproduce an original digital signal free from the nonlinear distortions.
2. Description of the Prior Art
It has been customary with a magnetic recording apparatus for digital signals to use a linear equalizer implemented by linear elements in removing distortions from the output of a read head. The linear equalizer is satisfactory in the case of signal reproduction with a low density digital signal storing apparatus. However, when it comes to signal reproduction with a high density storing apparatus, the conventional linear equalizer cannot remove distortions due to intersymbol interference since not only such distortions but also nonlinear distortions particular to the apparatus increase. Typical of nonlinear distortions is a peak shift, i.e., a shift of the peak of a data symbol read out on the time axis.
To equalize the read signal including the abovementioned distortions due to intersymbol interference and nonlinear distortions, use may be made of an equalizer using a Viterbi algorithm as proposed in G. David FORNEY, JR, "Maximum-Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interference, "IEEE Trans. Inform. Theory, vol. IT-18, No. 3, May, 1972. The approach applied to this equalizer consists of preparing presumed value signals containing distortions of various kinds of symbol sequences, and selecting as an equalized output symbol sequence a presumed symbol sequence having the smallest sum of errors between the sum of the presumed estimate signals and the symbol sequence of the input digital signal, i.e., signal read out on a trellis transition diagram.
While this approach is desirable for the equalization of digital signals containing nonlinear distortions, it is not feasible for the output of a digital VTR (Video Tape Recorder) or similar high density, high speed storing device since it involves agreat amount of calculations.
To reduce the amount of calculations, the above-stated equalizing method using the Viterbi algorithm (referred to as a Viterbi decoding method hereinafter) may be combined with the principle of a decision feedback equalizer to implement an intermediate characteristic (see Alexandra Duel--Hallen et al, "Delayed Decision-Feedback Sequence Estimation", IEEE Tran. commu. pp. 428, vol. 37, May, 1989). This scheme equalizes the former half of the impulse response including a precursor component by a procedure similar to the Viterbi equalizing method and equalizes the latter half by a procedure similar to a decision feedback equalizing method, thereby reducing the amount of calculations by a noticeably degree. However, the problem is that such a composite scheme is slightly inferior to the Viterbi decoding method with respect to equalizing ability and cannot remove distortions other than linear distortions.
There has also been proposed a decision feedback type equalizer capable of removing both the linear distortions and the nonlinear distortions (see Kevin Fisher, John Cioffi, and C. M. Melas, "An Adaptive DFE for Storage Channels Suffering from Nonlinear ISI," Globalcom 1989, 53.7.1). This type of equalizer can follow even the changes in distortions with respect to time by sequentially updating the contents of a RAM (Random Access Memory) with input digital signals. However, such an equalizer cannot remove nonlinear distortions from the precursor components of impulse responses due to the nature of the decision feedback equalizing method.